WO2004111718A1

WO2004111718A1 - Mach-zehnder electro-optical modulator configured to perform xor operation

Info

Publication number: WO2004111718A1
Application number: PCT/DK2004/000417
Authority: WO
Inventors: Jianfeng Zhang
Original assignee: Danmarks Tekniske Universitet
Priority date: 2003-06-16
Filing date: 2004-06-16
Publication date: 2004-12-23

Abstract

An electro-optical Mach-Zehnder modulator is configured to perform logic exclusive-or (XOR) or inverse exclusive-or (XNOR) operations. The invention may be applied in optical communication network components with a transmitter module used to convert electrical non-return-to-zero (NRZ) encoded signals to optical Manchester-encoded signals. The resulting optical signals are well suited for high bit rate (e.g. 40 Gb/s) operation, which needs no clock rate conversion. According to the invention, an electro-optical MZ modulator performs XOR or XNOR operations of electrical signals while simultaneously performing the electro-optical modulation of the signal onto an optical carrier signal.

Description

MACH-ZEHNDER ELECTRO-OPTICAL MODULATOR CONFIGURED TO PERFORM XOR OPERATION

FIELD OF THE INVENTION The present invention relates to electro-optical modulators such as Mach-Zehnder modulators. The invention provides a Mach-Zehnder electro-optical modulator configured to perform XOR operation for use in optical communication components. More specifically, the invention relates to a transmitter module used to convert electrical NRZ-coded signals to optical Manchester-coded signals.

BACKGROUND OF THE INVENTION

In telecommunication systems, there are several different digital signal encoding formats in use, including variations on unipolar, bipolar, nonreturn to zero (NRZ), return-to-zero (RZ), and Manchester encoding. There are also several different modulation schemes employed by the present systems, including amplitude-shift-keying (ASK), frequency-shift- keying (FSK) and phase-shift-keying (PSK). For optical communication systems, by far the most commonly used data waveform is NRZ in conjunction with amplitude on/off modulation. Almost all commercial transmission links over fiber optics at the bit rate varying from 155 Mb/s to 10 Gb/s have been employing NRZ coding due to its simple operation and efficient performance.

However, in recent years, researchers in optical communication field have found huge problems associated with NRZ format signal when it is exploited for further network applications, which are listed as follows, 1. One problem is overcoming fiber dispersion and nonlinearity impairments when the transmission link is upgraded to work at 40 Gb/s or higher bit rate.

2. Another problem is when network nodes employ semiconductor optical amplifiers

(SOA) or some other active semiconductor components, which usually exhibits pattern- dependent performance at high bit rates. 3. Another problem is in optical packet detecting when the packets' power or timing is greatly fluctuated, especially in a burst communication link.

For the first problem, researchers have found that RZ signal can greatly improve the transmission system's performance. To develop 40 Gb/s commercial fiber system, achievements have been made in generating RZ signal in special modulation formats, such as carrier-suppressed modulation, differential phase-shift-keying (DPSK).

For the last two presented problems, Manchester coding has proved an efficient solution. Manchester coding (MC) has been employed to reduce the waveform distortion in SOAs, which functioned as optical switching gates. Manchester packet receivers have also been reported to exhibit high-level fluctuation tolerance in a 10 Gb/s burst mode transmission link. In addition, Manchester coding is well suited for high bit rate operation, which needs no clock rate conversion. 40 Gb/s Manchester coding has been demonstrated by Murata et al. "Exclusive OR/NOR IC for >40Gbit/s optical transmission systems", Electronics Letters, 16, 764-765 (1998).

Future optical networks are expected to have optimized performance for bursty IP traffic. Due to its strengths listed above, Manchester will play an important role in future networks. Therefore, Manchester-related techniques and modules have great commercial potential.

Manchester and NRZ are different encoding formats, as indicated by Figure 1. In the NRZ code, a logic 'one' is defined as the high level state, while a logic 'zero' is defined as the low-level state. In the Manchester code, there is a transition at the middle of each bit period. The main bit transition serves as a clocking mechanism and also as data: a low-to- high transition represents 'one', and a high-to-low transition represents 'zero'. Such an encoding may alternatively be viewed as a phase encoding where each bit is encoded by a positive 90-degree phase transition, or a negative 90-degree phase transition. The Manchester code is therefore sometimes known as bi-phase code.

Manchester requires at least one transition in each bit period and may have as many as two transitions, thus its maximum modulation rate is twice for NRZ. Figures 2A and B show the power spectra of Manchester-coded and NRZ-coded random pattern signal at 10 Gb/s, respectively. Compared with NRZ, Manchester distributes more power in the high frequency region and less power in the low frequency region. A strong clock frequency component can also be observed in the spectrum.

Typical implementations of Manchester coding and decoding in the electrical regime are shown in Figures 3A and 3B. Generation of an electrical Manchester coded signal 33 is illustrated in Figure 3A. Here, a logic electrical component 32 is usually employed to perform an exclusive-or (XOR) operation between an original NRZ 30 and clock signal 31. Since no clock rate conversion is needed in this operation, Manchester coding can work well at high bit rate. The decoding of the Manchester signal 33 is illustrated in Figure 3B. Here, a differential amplifier 34 is usually employed to compare the energy received in each half time slot. The differential operation creates a power difference between consecutive half bits, thus the signal can be converted back to a signal 30 the NRZ format. Although Manchester coding requires double the bandwidth of NRZ coding, it has several advantages to compensate for this.

Simplicity in timing extraction: There is a predictable transition during each bit period, therefore, the timing extraction at the receiver for the Manchester code is much simpler than for the NRZ code. For this reason, the Manchester codes are known as self-clocking codes.

- Low requirements on components' frequency response: As indicated by the spectrum of Figure 2A, Manchester moves much signal power from the low frequency region to the high frequency region, thus it put low requirements on components' modulation performance at low frequency. In the case that some components exhibits long-bit- pattern dependent performance, Manchester coding can help to improve system performance.

- High tolerance to power fluctuation: The Manchester receiver usually employs a differential scheme to detect the signal. The differential receiver outputs the difference between the first half bit and the latter half bit, thus allowing the threshold voltage at the decision circuit to be fixed at zero volts independent of the input power. In this way, it has a large tolerance to the power fluctuation of the signal.

The advantages of Manchester coding have been throughout researched. It has been shown that Manchester could improve DFB lasers modulation performance in the coherent communication system, see e.g. Noe et al. "Optical FSK transmission with pattern independent 1119 photoelectrons/bit receiver and endless polarization control", Electronics Letters 25, 757-758 (1989). In a 10 Gb/s packet switched WDM network employing SOAs as switching gates, Manchester coding suppressed the waveform distortion of the optical signal and greatly improved the system performance, see e.g. Shibata et al.

"Semiconductor laser diode optical amplifiers/gates in photonic packet switching", Journal of Lightwave Technology 16, 2228-2235 (1998). Moreover, the research group in Stanford employed Manchester to implement signaling in WDM networks, see e.g. Ho et al., CLEO'98, CMG6, 1998. Manchester also enables operation of 10 Gb/s burst-mode packet receivers, which shows great tolerance to packet timing jitter and power fluctuation, see e.g. Nishizawa et al., Journal of Lightwave Technology 20, 1078-1083 (2002), Yamada et al., Journal of Lightwave Technology 16, 2220-2227 (1998), and Akahori et al., IEEE Photonics Technology Letters 10, 869-871 (1998).

The conventional method of generating an optical MC signal is shown in Figure 4. In this configuration, an electrical XOR gate 32 couples the incoming electrical NRZ encoded data signal 30 with an electrical clock signal 31 to form an electrical Manchester encoded data signal 33. Then, the electrical Manchester signal 33 is used to modulate a laser or an optical modulator 35, whereby the optical Manchester signal 36 is generated.

A large number of documents applies the conventional method of generating optical MC signals described in the above and shown in Figure 4, some of these are; US

2003/0025971; Yamada et al., Journal of Lightwave Technology 16, 2220 (1998); Murata et al., Electronics Letters 34, 764 (1998); Nishizawa ey al., Journal of Lightwave Technology 20, 1078 (2002).

It is a disadvantage of the conventional methods of generating optical MC signals that high-speed electrical logic components is required to perform the XOR operation prior to the modulation on the optical carrier.

Chang et al. (Journal of Optical Communications 21, 122 (2000)) reports a method that receives optical signal by using photo-diodes and then controls a directional coupler switch through converted photocurrents to perform XOR operation between two optical signals, hence an opto-electronic XOR gate.

The method of Chang et al. is based on different principle and components from the present invention. As it performs XOR operations for optical signals, its applications will also be different from that of the present invention, which generates optical XOR output for electrical signals.

US 5,315,422 discloses an all-optical logic element capable of performing XOR or XIMOR operation at ultrahigh speeds. The element consists of an all-optical Mach-Zehnder modulator where first and second control light beams P₁ and P₂ modulate a phase of the light in each branched waveguide of the modulator. The resulting output, P₀, will correspond to P₁ XOR P₂ or P₁ XNOR P₂, depending on the length of the branched waveguides. US 5,315,422 mentions the application of optical XOR logical elements to perform bit pattern matching for e.g. label recognition in optical transceiver modules.

US 5,315,422 proposes a method in the all-optical domain which is useful only for all- optical signal processing applications. However, if it was to be used for generating an optical XOR output from the electrical signals, it will require complex configurations. For example, in order to be applied in generation of optical MC data signals, the electrical clock and data signal must first be modulated onto optical signals. Hence, US 5,315,422 teaches that the data content of electrical data signals should be optically modulated twice, first when generating signals P₁ and P₂, secondly the performing the XOR operation. US 4, 950, 882 propose electro-optical XOR gates and US 4,928, 310 proposes a functional module (a code generator) based on the XOR gates presented by US 4,950882. The disclosed electro-optical XOR gates consist of a pairs of laser diodes connected back-to- back in parallel by a pair of input leads to which electronic logic signals are applied.

It is a disadvantage of the XOR gate described in US 4, 950, 882 and US 4,928,310 that it uses two laser diodes for generating the XOR output and therefore outputs an in-coherent light signal. To perform high speed XOR operation, it also requires high-speed laser diodes and complex configuration of driving circuits.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method for performing electro-optical

XOR modulation using a dual driven Mach-Zehnder electro-optical modulator.

It is another object of the present invention to provide a method for performing

Manchester encoding in the electro-optical regime, which does not require high-speed electrical logic components.

It is still another object of the present invention to provide an optical Manchester transmitter module applying a dual driven Mach-Zehnder electro-optical modulator configured to perform XOR logic operation in the electro-optical domain.

A first aspect of the invention addresses the problem of how to perform fast and efficient logic XOR or XIMOR operations of electrical signals to be used in optical networks.

According to the first aspect, the invention provides a method for performing logic exclusive-or (XOR) or inverse exclusive-or (XNOR) operations of electrical signals while simultaneously modulating the resulting signal onto an optical carrier, the method comprising the steps of: - providing a Mach-Zehnder interference type electro-optical modulator having a first arm with a first electrically driven phase modulating element and a second arm with a second electrically driven phase modulating element,

- providing different parts of an optical carrier signal in each arm of the modulator,

- receiving a first amplitude modulated electrical signal at the first phase modulating element,

- receiving a second amplitude modulated electrical signal at the second phase modulating element,

- setting a bias voltage between the first and the second phase modulating element, - modulating the amplitude modulation of the first electrical signal on to a phase of the optical carrier signal part in the first arm,

- modulating the amplitude modulation of the second electrical signal on to a phase of the optical carrier signal part in the second arm, - overlapping the phase modulated optical carrier signal part from the first arm with the phase modulated optical carrier signal part from the second arm to form an optical interference signal, and

- adjusting bias voltage and amplitudes of the first and second electrical signals so that the optical interference signal have an amplitude modulation at least substantially equal to

(amplitude modulation of the first electrical signal) XOR or XNOR (amplitude modulation of the second electrical signal).

Compared with the conventional methods for performing electro-optical XOR operation, the method according to the first aspect of the present invention has several strengths:

- no high-speed electrical logic components are needed. Therefore, the proposed optical module is more simple and robust;

- the invention applies a special configuration of an optical Mach-Zehnder modulator. Since DFB lasers integrated with a Mach-Zehnder modulator have been commercially available, the method according to the invention can provide compact modules with excellent performance;

- the generated optical signal can have a carrier-suppressed optical spectrum, implying its large tolerance to fiber nonlinearity and its capability of long haul transmission.

In order to perform a logic operation between the first and second electrical logic signals, the should have at least substantially the same high and low voltage levels, as they can not otherwise cancel each other. Assuming that they swing in the range [V_L, V_H], where V_L is the voltage at low logic level, and V_H is the voltage at high logic level, the bias voltage and amplitude of the electrical signals are preferably adjusted so that

V_1111n -V₇₁ < V_b -(V_H -V_L) < V_min < V_b + (V_H -V_L) < V_min +V_π or

V_max -V₁₁ < V_b -(V_H - V_L ) < V_max < V_b + (V_H -V_L) ≤ V_max + V_π

where V_b is the bias voltage, V_π is a swing voltage, V_mιn is a voltage difference between the phase modulating elements where the optical interference signal achieves an amplitude minimum, V_max is a voltage difference between the phase modulating elements where the optical interference signal achieves an amplitude maximum. A second and a third aspect of the invention addresses the problem of transforming electrical IMRZ encoded data signals into MC optical signals to be used in optical networks.

In the second aspect, the invention provides a method for converting a non-return-to-zero (NRZ) encoded electrical data signal to a Manchester encoded optical data signal, the method comprising the steps of

- providing a Mach-Zehnder interference type electro-optical modulator having a first arm with a first electrically driven phase modulating element and a second arm with a second electrically driven phase modulating element, - providing different parts of an optical carrier signal in each arm of the modulator,

- receiving a NRZ encoded electrical data signal at the first phase modulating element, receiving an electrical clock signal at the second phase modulating element,

- setting a bias voltage between the first and the second electrically driven phase modulating element, - modulating the NRZ modulation of the NRZ encoded electrical data signal on to a phase of the optical carrier signal part in the first arm, modulating the electrical clock signal on to a phase of the optical carrier signal part in the second arm, overlapping the phase modulated optical carrier signal part from the first arm with the phase modulated optical carrier signal part from the second arm to form an optical interference signal, and

- adjusting bias voltage and amplitudes of the NRZ encoded electrical data signal and the electrical clock signal so that the optical interference signal have a Manchester encoded amplitude modulation at least substantially equal to (NRZ signal) XOR or XNOR (clock signal).

In the third aspect, the invention provides an optical transmitter module for generating Manchester encoded optical data signals, the module comprising

- first input means for receiving an electrical data signal, - second input means for receiving an electrical clock signal,

- a Mach-Zehnder interference type electro-optical modulator having a incoming waveguide part for receiving an optical carrier signal, the waveguide being split into - a first arm with a first electrically driven phase modulating element connected to the first input means for receiving the electrical data signal, - and a second arm with a second electrically driven phase modulating element connected to the second input means for receiving the electrical clock signal, the first and the second arm being coupled to an outgoing waveguide part,

- a light source optically connected to the incoming waveguide part of the optical modulator for providing the optical carrier signal, and - output means connected to the outgoing waveguide part for receiving and transmitting the modulated optical carrier signal.

Preferably, the transmitter module further comprises - a first electrical amplifier connected to the first input means, for receiving and adjusting amplitudes of the electrical data signals to be received by the first input means, a second electrical amplifiers connected to the second input means, for receiving and adjusting amplitudes of the electrical clock signal to be received by the second input means.

In this case, the phase modulating elements are connected to the input means through the electrical amplifiers. Also, various other electrical components, such as capacitors and filters, may be connected in series or parallel between the phase modulating elements and the input means.

In a fourth aspect, the invention provides the use of an optical transmitter module comprising

- a Mach-Zehnder interference type electro-optical modulator having a incoming waveguide part for receiving an optical carrier signal, the waveguide being split into a first arm with a first electrically driven phase modulating element for receiving an electrical data signal and a second arm with a second electrically driven phase modulating element for receiving an electrical clock signal, the first and the second arm being coupled to an outgoing waveguide part,

- a light source optically connected to the incoming waveguide part of the optical modulator for providing the optical carrier signal, and

- first and second electrical amplifiers connected to the first and second electrically driven phase modulating elements, respectively, for receiving and adjusting amplitudes of the data signal and the clock signal, respectively, for converting a non-return-to-zero (NRZ) encoded electrical data signal to a Manchester encoded optical data signal.

Electro-optical MZ modulators have been one of the most common components in designing optical networks during the last ten years. Still, no one has ever reported applying the electro-optical MZ modulator to perform XOR or XNOR operations of electrical signals while simultaneously performing the electro-optical modulation of the signal onto the optical carrier.

None of the cited prior art references perform simultaneous XOR/XNOR operation and electro-optical modulation. All applies separate steps and components for the XOR/XNOR operation and the electro-optical modulation, thereby increasing system complexity and costs while typically reducing speed and performance.

It is to be understood that methods, apparatus' and uses which perform a non-perfect XOR or XNOR operation, or which produce a noisy, distorted or otherwise non-perfect MC signal, applying the principles of the present invention also falls within the scope of the present application.

BRIEF DESCRIPTION OF THE FIGURES Figure 1 illustrates waveform and eye-diagram for NRZ and Manchester encoded data signals.

Figures 2A and 2B show power spectra of NRZ and Manchester encoded random pattern signals at 10 Gb/s, respectively.

Figure 3A and 3B illustrates typical implementations of electrical Manchester coding and decoding, respectively.

Figure 4 illustrates a conventional optical Manchester transmitter.

Figure 5 is a diagram illustrating a setup for generating optical Manchester-coded signals according to an embodiment of the invention.

Figure 6A and B are graphs showing the optical intensity as a function of the difference in phase between the two arms of the MZ modulator, the graphs illustrate the typical configuration of a dual arm MZ modulator.

Figure 7A and B are graphs showing the optical intensity as a function of the difference in phase between the two arms of the MZ modulator, the graphs illustrate the XOR configuration of a dual arm MZ modulator.

Figure 8A - D are graphs showing the waveforms (C and D) resulting from an XOR operation between NRZ data and clock (A and B).

Figure 9A and B are eye diagrams (100 ps/div) of 2.5 Gb/s optical MC signals with (A) V_bias=0 and (B) V_bias=V_π.

Figure 1OA and B are eye diagrams (25 ps/div) of 10 Gb/s optical MC signals with (A) V_bias=0 and (B) V_bias=V_π. Figure HA and B are eye diagrams of electrically filtered optical MC signals with (A) 2.5 Gb/s (100 ps/div) and (B) 10 Gb/s (25 ps/div).

Figure 12 is a graph showing the measured optical spectrum of the generated MC signal.

Figure 13 is a graph showing the measured electrical RF spectrum of the generated 10 Gb/ s MC signal.

Figure 14 shows the setup for an experimental measurement of BER curves for the generated Manchester signal.

Figure 15A and B are eye diagrams of (A) the generated Manchester signal and (B) the decoded data signal resulting from the generated Manchester signal.

Figure 16 is a graph showing BER curves for a generated Manchester signal and an NRZ signal.

Figure 17 illustrates a transmission link in an example network.

Figure 18A and B are eye-diagrams of (A) optical NRZ payload at 10 Gb/s (25 ps/div) and (B) detected label at 622 Mb/s (500 ps/div).

Figure 19A and B are eye-diagrams of (A) optical MC payload at 10 Gb/s (25 ps/div) and (B) detected label at 622 Mb/s (500 ps/div).

Figure 2OA and B show RF spectra of the detected labels in (A) a signal with NRZ payload and (B) a signal with MC payload.

DETAILED DESCRIPTION OF THE INVENTION

The invention implements the Manchester coding function in the optical domain. As indicated by the technical background description earlier in the application, Manchester coding complements the NRZ coding and improves the system performance in many network applications.

A schematic diagram of an optical Manchester transmitter module 49 according to the invention is shown in Figure 5. The main component in the transmitter module is a dual- driven Mach-Zehnder modulator 50. The modulator has an input waveguide 52 which is split into a first arm 53 and a second arm 54. The input waveguide 52 receives an optical signal 51 from a continuous-wave (CW) laser 60. The optical signal 51 is the optical carrier signal. Each arm has an electrically driven phase modulating element 55, typically a section of the waveguide arms 53, which modulates the refractive index in response to a received electrical signal. The modulators are fed with an electrical NRZ data signal 30 and an electrical clock signal 31. Amplifiers 58 adjust the amplitudes of signals 30 and 31 and filters 64 remove any DC components, resulting in electrical NRZ and clock signals 62 and 63. The phase modulating element 55 of the first arm 53 receives the electrical IMRZ data signal 62, and modulates the amplitude modulation of the NRZ signal 62 on to a phase of the optical signal in the arm 53. The phase modulating element 55 of the second 54 receives the electrical clock signal 63 and, equivalently, transfers this electrical amplitude modulation into a phase modulation on the optical signal propagating through the element 55. The waveguide arms 53 and 54 couples again in the outgoing waveguide 56. In this coupling, the phase modulated optical signals from the waveguide arms 53 and 54 are overlapped to form an optical interference signal 57. When the bias V_b difference of this modulator is set to be zero or swing voltage V_π, the modulator will perform XOR or inverse XOR operation between the electrical input signals 30 and 31. Hence the optical interference signal 57 will have an amplitude modulation equal to:

(amplitude of electrical NRZ signal 30) XOR (amplitude of electrical clock signal 31).

The result being a modulation of the input CW light 51 and generating the Manchester- coded (MC) optical signal 57 carrying the same data as the electrical NRZ data signal 30.

The transmitter module 49 will typically be used in a network component, such as a router or a switch, to generate a Manchester coded optical signal to a fiber network. Such a module can be used to construct a high-speed burst mode transmission link, or to improve the modulation performance of network nodes in certain cases. The most important interfaces to the network component are therefore electrical input means 65 for receiving the electrical NRZ data signal, electrical input means 66 for receiving the electrical clock signal, and optical output means 67 connected to the outgoing waveguide 56 for transmitting the generated Manchester signal to the optical network.

To understand the special configuration of the MZ modulator used for XOR operation and Manchester encoding according to the invention, we must first look upon the typical configuration of a dual arm MZ modulator, that is a MZ modulator with two varying modulation voltages. The basic concept behind the MZ modulators involves the use of a MZ interferometer design to modulate the optical signal. A MZ interferometer relies on the interference effects of light. The classic MZ interferometer uses mirrors to split a light beam into two equal paths that are recombined at the end, interfering constructively. By inducing a phase shift on one branch, the light can be made to interfere destructively. In a MZ modulator, splitters replace the mirrors and the branches are typically planar waveguides. The phase shift is induced by changing the index of refraction of a section of the waveguide with an electric field. Thus, a voltage is applied to bring the total phase shift between the two arms of the interferometer to - 0 resulting in constructive interference and maximal transmission, or - π resulting in destructive interference and minimal transmission.

The amount of voltage needed to shift the phase in one branch by π radians in relation to the other arm is denominated V_π or the swing voltage. In the ideal condition, when no voltage is applied, the phase shift between the two branches of the interferometer should be equal to zero, causing the light to interfere constructively and resulting in maximum transmission. However, there may exist some residual electrical field or path length difference between the branches. Therefore, in practice, a small bias voltage may also be needed to achieve the zero phase shift.

Usually, the phase shift induced has a linear relation with the applied voltage. In other words, a voltage with a value V, will induce the phase shift (π • V/V_π).

Considering a CW light input and a MZ modulator with both arms modulated by electrical signals V₁ and V₂, the output should be the combination of two corresponding optical fields E₁ and E₂,

E₀₁₁₁ (O = E, (0 + E₂ (O

E₁ (t) cc eχp(iωt + π - V_λ (t)I V_π ) _ _{{ 1)} E₂ (t) ∞ exp(iωt + π - V₂ (t)/V_π)

Where V₁Ct) and V₂(t) are the input voltages to the phase modulating elements of the MZ modulator. To be more specific, we assume V₁(I:) is composed by a DC offset V_b and AC coupled voltage V₁O:), whereas V₂(t) is AC coupled to the other arm (i.e. the DC component of V₂(t) is blocked), that is

V₂(t) = v₂(t) Where v^t) and v₂(t) are denoted the modulation voltages. In practice, we can control the bias voltage V_b and the voltage amplitude of V^t) and V₂(t) in many ways. For instance, a bias-tee can be used to AC couple the input voltage into the modulator's arms while providing a DC bias control. Some MZ modulators even have a bias-tee inside, hence providing both DC bias port and modulation ports.

The optical intensity of the output should be

By integrating Equations (1) and (2) into (3), we can get

/„.., oc cos' |^~-⁽n +^V ₁W-^V ₂(O) (4)

Turning to Figure 5, 1₀ denotes the intensity of the optical CW signal 51, V_b denotes bias the voltage difference between the two arms 53 and 54, Vj(t) denotes the modulation voltage 62 in arm 53, and v₂(t) denotes the modulation voltage 63 in arm 54. Thus, it is the differences in the induced phase shift in each arm, which is proportional to V₁Ct)-V₂Ct), that determines the modulation.

The operation of a typically configured dual arm MZ modulator is illustrated in the following.

In a dual arm MZMs, both v^and v₂(t) carry the modulated data information, and they have conjugate relations. In the single driven case, one of the modulation voltages will be constant while the other modulation voltage carry data information. The desired modulation of the optical output is I_out(t) oc Iv₁Ct) - v₂(t) |.

The bias voltage and the modulation voltages should be set to make the modulated phase difference between the two arms (that is, (V_b + V₁Ct) - v₂(t) ) swing in a range [2nπ,

2nπ+π], or similarly in a range [2nπ+π, 2nπ+2π], where n is an integer. This is illustrated in Figure 6A (phase difference swings in [2nπ, 2nπ+π]), and 6B (phase difference swings in [2nπ+π, 2nπ+2π]).

To achieve the highest on-off extinction ratio, the bias voltage and the amplitude of the modulation voltages should be set to make the minimum and maximum values of the phase difference between the two arms equal to (2nπ and 2nπ+π) or (2nπ+π and 2nπ+2π), respectively.

The XOR configuration of the MZ modulator is also based on equation (4). In this case, the desired modulation of the optical output is I_out(t) oc v_α(t) XOR v₂(t) or I_out(t) ∞ V₁Ct) XNOR v₂(t).

The XOR configuration of the MZ modulator is some relations that the bias voltage and the modulation voltages should fulfil. For illustration purposes, we can start with the reasonable assumption that

V₁ - V₂ < V_^ (5)

Equation (4) now gives the following table,

Table 1. XOR/XNOR operation of a Mach-Zehnder modulator

From table 1, we can conclude that the specially biased Mach-Zehnder modulator 50 can perform a logic operation between the electric signals and generates the corresponding optical signal. In this sense, the Mach-Zehnder modulator 50 acts as an electro-optical XOR gate. Such a characteristic can be utilized to implement the optical Manchester transmitter.

To be more practically, the relations of the bias voltage and driven voltage in the XOR configuration are described in detail in the following.

Assuming that the MZ modulator achieves the minimum transmission with a voltage difference Vm_In between the phase modulating elements, and the maximum transmission with a voltage difference V_max, V_min and V_max will have the following relation,

V_max - V₁ min = V. (6)

After AC coupling, the modulation voltages swings in the ranges V₁Ct) ε [V_1L,V_1H], and v₂(t) € [V_2L, V_2H- - Then, to generate an optical signal with symmetrical eye-diagrams, the following relation is preferably fulfilled.

where we have also defined the values V_H and V_L. To perform the operations related XOR, the bias voltage, V_b, should be set to meet the following relations,

V_m!n -V_π < V_b - (V_H -V_L) < V_min < V_{b +} (V_H -V_L) < V_{mιn +}V_π or (8)

V_raaλ -V, < V_b - (V₁₁ -V_L) < V_max < V_b + (V_H -V_L) < V_max + V_π

To achieve the maximum on-off extinction ratio, the following equation should be further met,

V_b = V_min and V_H -V_L = V_π or (9)

V_b = V_max and V_H -V_L = V_π

To get the best modulation performance, i.e. the maximum extinction ratio and symmetrical eye-diagrams, the relations of Equations (7)-(9) should be fulfilled.

This is illustrated in Figure 7 A (phase difference swings around 2nπ), and 7B (phase difference swings around 2nπ+π).

As an example, the bias voltage can be adjusted so that the phase difference between arml and arm2 is zero when no signal is input. When a voltage equal to V_π is input into arml, the phase difference become π (arml-arm2=π); when a voltage equal to V_π is input into arm2, (no voltage input into arml), then the phase difference become -π (arml- arm2=-π); when V₇₁ is simultaneously input into both arms, the phase difference will be zero again.

In other words, the bias voltage and the modulation voltages should be set to make the MZ modulator operate in two regimes. One pulse input into one arm will push operation into a first regime, while a pulse input the other arm will pull operation into a second regime. When no pulse are input, or when pulses are input into the two arms simultaneously, the operation should fall close to the boundary between the two regimes, preferably just at the boundary (2nπ or 2nπ+π).

To obtain symmetrical eye diagrams and maximum on-off extinction ratio, the following further requirements on the modulation signals and the bias voltage can be set: 1) The modulation signals should have the same amplitude (peak-to-peak), which is equal to swing voltage Vπ.

2) The bias voltage should be set to make the minimum and maximum values of the phase difference equal to (2nπ and 2nπ+2π) or (2nπ-π and 2nπ+π), balanced at 2nπ+π or 2nπ, respectively.

The XOR operation described in the above can be used to perform encoding translation from NRZ to Manchester coding, while simultaneously performing the electro-optical modulation of the signal onto the optical carrier. In this case, v^t) is an electrical NRZ data signal and v₂(t) is an electrical clock signal. The resulting modulation on the optical signal corresponds to an XOR (or XNOR) logical operation between the NRZ signal and the clock signal, being the Manchester encoding of the binary data of the NRZ signal.

In most cases, the NRZ data signal is DC-balanced through coding or scrambling, and the clock signal has a 50% duty ratio.

According to equation (9), to get the best performance, the peak to peak amplitude of electrical NRZ data and clock signal should be equal to the swing voltage of the modulator, while the bias voltage should be at the point where the maximum or minimum transmission occurs.

According to the descriptions listed above, we can see the difference between the NRZ operation and XOR/Manchester operation is that

1. In NRZ operation, the bias and modulation voltages co-operate to make the MZ modulator work in one phase regime.

2. In XOR/Manchester operation, the bias and modulation voltages co-operate to make the MZ modulator work in two adjacent phase regimes.

The logic operation of the modulator is validated through experimental measurements presented in Figure 8A-D. In the experiment, an optical modulator with 15 GHz modulation bandwidth is used, the operation bit rate can vary from 2.5 Gb/s to 10 Gb/s. An electrical NRZ signal, Figure 8A, and an electrical clock signal, Figure 8B, is received by an optical Manchester transmitter module 49 as described in relation to Figure 1. The output waveforms shown in Figures 8C and D shows that the bit information is finally transformed into the transition edge at the middle of each bit period.

Figure 9A-B and Figure 10A-B show the eye-diagrams of the optical MC signals of Figure 8C and D (2.5 Gb/s and 10 Gb/s) for different values of V_b. The large eye-opening and the sharp transition edge at the middle of the bit period verify this module's good performance. In the shown eye-diagram, sharp transition edges can also be observed at the boundary of the bit period, which is caused by some particular bit patterns. When the optical signal is detected, those edges can be equalized by an electrical filter, which should not degrade the receiving performance. The electrical filtered signals are shown in Figures HA and B.

The measured optical wavelength spectrum of the generated MC signal is shown in Figure 12. When the modulator is biased at V_π, an optical MC signal with the carrier-suppressed spectrum is generated. Such a signal has large tolerance to the non-linear effects and dispersion in fiber, thus suitable for long distance transmission. The measured electrical RF spectrum of the generated 10 Gb/s MC signal is shown in Figure 13.

An experimental set-up for measuring the BER performance of the proposed Manchester coding method is shown in Figure 14. In the setup, a IMRZ data generator 81 inputs a 10 Gb/s NRZ data signal 30 into a Mach-Zehnder modulator 50 through amplifier 58.. Similarly, a clock generator 81 inputs a 10 GHz clock signal 31 into the Mach-Zehnder modulator 50 through amplifier 58. The Mach Zehnder modulator 50 is configured to perform the Manchester coding of the input data signal 30 on an optical CW signal from a laser 60. The generated data NRZ is a pseudo-random data stream with 2³¹-1 pattern length.

The generated optical Manchester coded signal 57 is received by a 20 GHz bandwidth optical receiver 82. With the aid of a 20 Gb/s electrical demux module 83, the optical Manchester coded signal 57 can be decoded. The recovered data signal is then input into the error detector 84 for the BER measurement. In the whole setup, the 10 GHz clock is also used to synchronise the 20 Gb/s electrical demux module 83 and the error detector 84.

Figure 15A and B show eye diagrams of the output of the optical receiver 82 and of the 20 Gb/s demux module 83, respectively. As indicated by the large eye-opening of the eye- diagrams, the quality of the generated Manchester-coded signal is quite good. Graph 90 of Figure 16 shows a Bit Error Ratio (BER) data 91 of the optical Manchester coded signal 57. It can be seen that the Manchester coding method has an error-free performance and that the receiver sensitivity at BER=IO^"9 is about -20.5 dBm. Data 92 shows the BER performance of the same optical receiver input with a 20 Gb/s NRZ signal. Comparison between data 91 and 92 shows that the power penalty induced by the Manchester coding is about 3dB. However, the receiver sensitivity could be further improved with a differential detection scheme. The BER measurement result verifies that the proposed Manchester coding method has a stable and error-free performance.

In the following sections, an application example is given to verify the performance of the present invention. It is shown that upgrading of a network transmission link with the present invention can greatly improve the performance of the network.

The network of the example employs a novel optical label switching scheme to improve network throughput and realize efficient network management. Its basic transmission link is shown in Figure 17. The network processor 86 receives the IP packets 85 and then aggregates them into an electrical payload signal 87. For each payload signal, a label signal 88 is generated to indicate its destination address. Payload signal 87 is fed to the payload transmitter 89 operating at 10 Gb/s, an optical payload is therefore generated. Then the FSK transmitter 90 modulates the label information 88 onto the light, hence forming an optical payload tagged with an optical label. The optical signal is amplified by an optical amplifier 91 and then injected into an 80 km fiber transmission link 92. At the receiving end, an optical splitter 93 splits the optical signal into two branches. At one branch, the label is first demodulated by a FSK demodulator 94, and then detected by a photodiode 97. The label receiver 95 will process the label information for network routing functions. At the other branch, the optical payload can be directly detected and recovered by the payload receiver 96.

In the network of the example, both the payload and label are usually in the NRZ format, i.e. the payload transmitter 89 is an optical NRZ transmitter. However, it is shown that the intensity modulation of the payload results in the power fluctuation of received label, thus degrading the network performance. Figure 18A shows an eye-diagram of an optical NRZ payload and Figure 18B shows an eye-diagram of an optical label having a NRZ Payload. With 13dB-extinction-ratio optical NRZ payload, the detected label eyes are nearly closed. The origin of this crosstalk is attributed to overlapping between spectra of the label and payload, as indicated by Figure 19A showing a RF spectrum of a label having a NRZ payload. To compromise the system performance, the extinction ratio of payload has to be around 4 dB. This put great limitations to the network.

As Manchester coding has unique characteristics of good DC-balance, it can shape the payload spectrum and improve the receiving performance. By replacing the optical NRZ transmitter with an optical Manchester transmitter according to the present invention operating at 10 Gb/s, the system performance is greatly improved. Figure 2OA shows an eye-diagram of an optical MC payload and Figure 2OB shows an eye-diagram of an optical label having a MC Payload. As can be seen from Figure 2OB, the detected label has a large eye-opening when it has a 13dB-extinction-ratio MC payload generated according to the invention. Hence, much less crosstalk is observed in Figure 19B showing a RF spectrum of a label having a MC payload. In this way, the limitations on extinction ratio can be removed.

Claims

1. A method for performing logic exclusive-or (XOR) or inverse exclusive-or (XNOR) operations of electrical signals while simultaneously modulating the resulting signal onto an optical carrier, the method comprising the steps of: providing a Mach-Zehnder interference type electro-optical modulator having a first arm with a first electrically driven phase modulating element and a second arm with a second electrically driven phase modulating element, - providing different parts of an optical carrier signal in each arm of the modulator,

- receiving a second amplitude modulated electrical signal at the second phase modulating element, - setting a bias voltage between the first and the second phase modulating element,

- modulating the amplitude modulation of the first electrical signal on to a phase of the optical carrier signal part in the first arm,

2. A method according to claim 1, wherein the step of adjusting the bias voltage and amplitudes of the first and second electrical signals comprises adjusting these to so that

V_rain - V, < V_b - (V_H -V_L) < V_min < V_b + (V_H -V_L) < V_n,, +V_π or

V_max - V, < V_b - (V_H - V_L) ≤ V_max < V_b + (V_H - V_L) < V_max + V_π

where V_b is the bias voltage, V₁₁ is a swing voltage, V_min is a voltage difference between the phase modulating elements where the optical interference signal achieves an amplitude minimum, V_max is a voltage difference between the phase modulating elements where the optical interference signal achieves an amplitude maximum, and where each of the first and second electrical signals swing in the range [V_L, V_H], where V_L is the low level voltage and V_H is the high level voltage.

3. A method for converting a non-return-to-zero (NRZ) encoded electrical data signal to a Manchester encoded optical data signal, the method comprising the steps of

- providing a Mach-Zehnder interference type electro-optical modulator having a first arm with a first electrically driven phase modulating element and a second arm with a second electrically driven phase modulating element,

- providing different parts of an optical carrier signal in each arm of the modulator, - receiving a NRZ encoded electrical data signal at the first phase modulating element,

- receiving an electrical clock signal at the second phase modulating element,

- setting a bias voltage between the first and the second electrically driven phase modulating element,

- modulating the NRZ modulation of the NRZ encoded electrical data signal on to a phase of the optical carrier signal part in the first arm,

- modulating the electrical clock signal on to a phase of the optical carrier signal part in the second arm,

- overlapping the phase modulated optical carrier signal part from the first arm with the phase modulated optical carrier signal part from the second arm to form an optical interference signal, and

- adjusting bias voltage and amplitudes of the NRZ encoded electrical data signal and the electrical clock signal so that the optical interference signal have a Manchester encoded amplitude modulation at least substantially equal to

(NRZ signal) XOR or XNOR (clock signal).

4. A method according to claim 3, wherein the step of adjusting the bias voltage and amplitudes of the NRZ encoded electrical data signal and the electrical clock signal comprises adjusting these to so that

V_min - V_n < V_b - (V_n -V_L) ≤ V_min < V_b + (V_H - V_L ) < V_min + V_π or

V_max -V, < V_b -(V₁₁ -V_L) < V_max < V_b + (V_H - V_L) < V_max + V_π

where V_b is the bias voltage, V₇₁ is a swing voltage, V_min is a voltage difference between the phase modulating elements where the optical interference signal achieves an amplitude minimum, V_max is a voltage difference between the phase modulating elements where the optical interference signal achieves an amplitude maximum, and where each of the electrical NRZ signal and clock signal swing in the range [V_L, V_H], where V_L is the low level voltage, and V_H is the high level voltage.

5. An optical transmitter module for generating Manchester encoded optical data signals, the module comprising

6. An optical transmitter module according to claim 3, further comprising

- a first electrical amplifier connected to the first input means, for receiving and adjusting amplitudes of the electrical data signals to be received by the first input means,

- a second electrical amplifiers connected to the second input means, for receiving and adjusting amplitudes of the electrical clock signal to be received by the second input means, and wherein the first and second phase modulating elements are connected to the first and second input means through the first and second electrical amplifiers.

7. The use of an optical transmitter module comprising

- a Mach-Zehnder interference type electro-optical modulator having a incoming waveguide part for receiving an optical carrier signal, the waveguide being split into a first arm with a first electrically driven phase modulating element for receiving an electrical data signal and a second arm with a second electrically driven phase modulating element for receiving an electrical clock signal, the first and the second arm being coupled to an outgoing waveguide part, - a light source optically connected to the incoming waveguide part of the optical modulator for providing the optical carrier signal, and